Across the entire gamut of technology of optical information processing, in order to realize high-density optical recording, a blue light laser stably oscillating blue light with a wavelength of approximately 400 to 430 nm at an output of 30 mW or more is required, and a development race is now on. As a blue light source, there is expected an optical waveguide type wavelength conversion device in which a laser that oscillates red light as a fundamental wave and a second-harmonic-generation device of a quasi-phase-matched system are combined.
Nonlinear optical crystal such as lithium niobate or lithium tantalate single crystal has a high secondary nonlinear optical constant. When a periodic domain polarization inversion structure is formed in the above crystals, a second-harmonic-generation (SHG) device of a quasi-phase-matched (QPM) system can be realized. Further, when a waveguide is formed within this periodic domain inversion structure, the high-efficiency SHG device can be realized and further, applied to optical communication, medical science, photochemistry, various optical measurements over a wide range.
In IQEC/CLEO-PR 2005, Tokyo, Japan, Jul. 11-15, 2005, post-deadline paper PDG-2, disclosed is a technology in which a fundamental wave from a DFB laser diode is condensed by a condenser lens, higher harmonics (green light) are obtained by irradiating the wave onto a PPLN optical waveguide device, and these higher harmonics are condensed to thereby be oscillated. The PPLN optical waveguide device is obtained by forming an optical waveguide in an MgO doped lithium niobate single crystal substrate and forming the periodic domain inversion structure within this optical waveguide. An incident side end face and projection side end face of the optical waveguide device made of lithium niobate single crystal are polished so as to be largely inclined from a plane perpendicular to the optical waveguide. Thereby, the incidence of return light onto a laser oscillation source is prevented.
Further, the present assignee disclosed that the periodic domain inversion structure is formed on a bulk substrate including a Z plate made of a lithium niobate single crystal and the second-harmonic-generation device is manufactured (Japanese published unexamined application No. 2005-70192A). Also, the assignee disclosed that a channel type optical waveguide is formed on a thin lithium niobate single crystal-5 degree off-cut Y substrate, and the periodic domain inversion structure is formed within this optical waveguide to thereby obtain the second-harmonic-generation device (WO 2006/41172 A1).